Molecular sieves are classified by the Structure Commission of the International Zeolite Association according to the rules of the IUPAC Commission on Zeolite Nomenclature. According to this classification, framework-type zeolites and other crystalline microporous molecular sieves, for which a structure has been established, are assigned a three letter code and are described in the Atlas of Zeolite Framework Types, 6th edition, Elsevier, London, England (2007). Deca-dodecasil 3R is one of the molecular sieves for which a structure has been established and materials of this framework type are designated as DDR.
DDR framework-type molecular sieves have pores which are defined by intersecting channels formed by 8-membered rings of tetrahedrally coordinated atoms and which have cross sectional dimensions of ˜3.6 Å by ˜4.4 Å. DDR framework-type zeolites can therefore be potentially useful in sorptive separations, for example of methane from carbon dioxide, and in catalyzing chemical reactions, including the conversion of oxygenates to olefins (OTO), where small pore size is desirable.
One example of a DDR framework-type molecular sieve is ZSM-58. U.S. Pat. No. 4,698,217 describes a method for synthesizing ZSM-58 having a silica to alumina molar ratio of 50 to 1000 using a methyltropinium salt as the directing agent.
U.S. Patent Application Publication No 2009/0111959 discloses the synthesis of a DDR framework-type molecular sieve having a composition, in its calcined, anhydrous form, involving the molar relationship (n)X2O3:YO2, wherein X is a trivalent element, Y is a tetravalent element and n is from 0 to less than 0.01 and wherein the crystals of said material have an average diameter less than or equal to 2 microns. The material is synthesized in a fluoride-containing medium in the presence of an N-ethyltropanium compound as directing agent.
To date, DDR-type molecular sieves are relatively underdeveloped, partly because of the high cost of their synthesis, especially the structure directing agent, and partly because of the difficulty of controlling the size and uniformity of the crystals produced. For example, for kinetically selective separation processes, it is desirable that molecular sieve crystals employed as the sorbent have a large crystal size and a narrow crystal size distribution. However, many existing synthesis processes produce DDR-type crystals having a wide size distribution or crystals that are highly overgrown and/or intergrown. There is therefore a need for new process of synthesizing DDR framework-type molecular sieves of reduced cost and/or that produce larger, more uniform crystals.